1. Field of the Invention
The present invention relates to a linear amplification with nonlinear components (LINC) power transmitter, and more particularly, to a LINC power transmitter which has better linear characteristics, higher efficiency, and less DC power consumption by forcefully saturating a main power amplifier and controlling the output of the main power amplifier to be at any desired level through an adjustment of a DC bias voltage of the main power amplifier.
2. Description of the Related Art
Even though it is common knowledge among those skilled in the art that efficiency and linearity are the most important factors that need to be considered in the manufacture of a power transmitter for a CDMA-type wireless terminal, many people still think that it is almost impossible to meet two requirements of a power transmitter for a CDMA-type wireless terminal, i.e., high efficiency and high linearity, at the same time based on the notion that efficiency is a trade-off for linearity and vice versa.
Due to the characteristics of a wireless terminal, a power transmitter generally outputs power much lower than its maximum capability. What really matters in transmitting power more efficiently is not the efficiency of a power transmitter at higher power levels but the efficiency of the power transmitter at lower power levels.
Even though a variety of techniques of enhancing efficiency and linearity at the same time have been adopted in conventional power transmitters for wireless terminals, the conventional power transmitters still seem to have a long way to go to be competitive enough in terms of achieving high efficiency even in a lower power range. A conventional linear amplification with nonlinear components (LINC) power transmitter shown in FIG. 1 is definitely not an exception to this notion.
As compared with typical power transmitters, the conventional LINC power transmitter of FIG. 1 can be considered almost perfect in terms of linearity but poor in terms of efficiency, in particular, at lower power levels. In the conventional LINC power transmitter of FIG. 1, bias of power amplifiers 62 and 64 is fixed to a predetermined voltage level by a fixed voltage power supplier 80. In addition, the output power level of the conventional LINC power transmitter of FIG. 1 is determined depending on gain variations in variable gain amplifiers (VGA) 51 and 52. Therefore, it is almost impossible for the convention LINC power transmitter of FIG. 1 to maintain high efficiency over a wide range of power levels. In FIG. 1, reference numeral 10 represents a digital signal processor, reference numeral 12 represents a signal component separator, reference numerals 21 through 24 represent digital/analog converters (D/A), reference numerals 31 through 34 represent low pass filters (LO), reference numerals 42 and 44 represent quadrature modulators, reference numeral 46 represents a local oscillator, reference numerals 53 and 54 represent gain amplifiers, represent numeral 60 represents a power amplification module, and reference numeral 70 represents signal combiner. The elements of the conventional LINC power transmitter of FIG. 1 are well known to those skilled in the art, and thus their detailed description will not be presented here in this disclosure.